Treatment of cellulosic organic acid esters



Patented Feb. 26, 1946 UNITED STATE s PATENT OFFICE TREATMENT OF CELLULOSIC ORGANIC ACID ESTERS No Drawing. Application November 17, 1943, Serial No. 510,657

11 Claims.

This invention relates to a, method of reducing the solution viscosity of cellulose organic acid esters, and more particularly the invention relates to the reduction of the solution viscosity of cellulose organic acid esters, such as cellulose acetate, which have been produced by the acylation of wood pulp cellulose.

In the production of cellulose organic acid esters, such as cellulose acetate, cellulose propionate, etc., for use in the manufacture of yarns, film and the like, it has been the well nigh universal practice heretofore to employ cellulose derived from cotton linters rather than the less expensive cellulose derived from wood pulp for the reason that the esters of wood pulp cellulose when substituted for esters of cellulose derived from cotton linters have always yielded inferior products. It has been found, however, that if wood pulp cellulose is esterified to such an extent that the solution viscosity of the resulting ester is substantially higher (e. g., twice as high) than the solution viscosity of the ester of cellulose derived from cotton linters, the difference in quality of the products produced therefrom largely disappears. For example, a yarn spun from a 24% concentration in acetone of wood pulp cellulose acetate, which solution has a solution viscosity of 700 poises, has about the same tenacity per denier and is comparable in other respects to a yarn spun from a cotton linter cellulose acetate solution of the same concentration which has a solution viscosity of only 350 poises, whereas a yarn spun from a 350 poise wood pulp cellulose acetate solution of the same concentration has a much lower tenacity. Obviously the more viscous solution is difiicult to handle and is generally disadvantageous from the technological standpoint, and accordingly means have been sought for reducing the solution viscosity of cellulose organic acid esters without otherwise afiecting their essential characteristics, including stability.

It is therefore an object of this invention to treat cellulose organic acid esters whereby to reduce their solution viscosities without impairing their desirable characteristics. A further object is to treat cellulose organic acid esters whereby to reduce their solution viscosities and preserve or increase their stability. A still further object is to treat freshly formed wood pulp cellulose acetate to decrease its solution viscosity and to improve its stability. Another object is to provide an improved step in the manufacture of cellulose organic acid esters and especially in the manufacture of acetates of cellulose derived from wood pulp. Other objects will be apparent from the following description.

These objects are realized by my invention which, briefly stated, comprises washing the freshly precipitated cellulose organic acid ester containing combined mineral acid with hard water acidified preferably with the same mineral acid as that combined with the cellulose ester to lower the pH of the wash water to not below approximately 4.5.

Sulfuric acid is commonly used as the catalyst for acetylation and, accordingly, sulfuric acid will be added to the wash water in sufficient quantities to give a pH of between 5.5 and 4.5, and preferably about 4.5.

Almost any ground or tap water (filtered and clarified) may be used, providing it has a hardness calculated as calcium carbonate of from 20 to 500 or more parts per million. Water that contains from 60 to 90 parts per million of hardness calculated as calcium carbonate is preferred.

The practice of my invention is illustrated by the following example wherein parts and percentages are by weight unless otherwise indicated.

EXAMPLE Cellulose acetate, prepared in the usual manner by the acetylation of wood pulp cellulose with acetic acid anhydride in acetic acid and in the presence of sulfuric acid as a catalyst, after saponifying to about 54.5% combined acetic acid, precipitating and washing with dilute acetic acid until the effluent wash liquor contains in the neighborhood of 1% acetic acid. is further washed with hard water prepared as follows. A filtered ground water containing about 75 parts: per million of hardness calculated as calcium carbonate 7 and having an initial pH of 7.8 is acidified with sulfuric acid to a pH of 4.5, and the cellulose acetate, substantially freed of acetic acid, is thoroughly washed over a period of about 20 hours with this water. The washed cellulose acetate is then drained, de-watered and air-dried. When dissolved to'form a 24% solution in acetone, the solution viscosity was found to be 345 poises. This cellulose acetate had excellent stability even though the ash alkalinity was low, being 0.69 l0- In comparison, a cellulose acetate prepared in the same way except that it was washed with the filtered ground water without the addition of acid and, otherwise processed in the same manner, gave a 24% acetone solution viscosity of .740 poises. Furthermore, the stability was not quite as good, even though in this case the ash alkalinity was more than. twice as much, being 1,4'IXl0- By making a number of similar tests, varying the acidity of the water, the following data were obtained:

It should be noted by reference to this table that when working with a fairly high viscosity cellulose acetate made from wood pulp cellulose, the viscosity drops very rapidly as the pH of 4.5 is approached. It is to be noted too that slightly acidified water within the range from a pH of 4.5 to a pH of 5.5 gives noticeably improved stabiliy as compared with cellulose acetate washed with untreated water, and that below a pH of 4.5 the stability decreases sharply. The change in ash alkalinity is also directly proportional to the lowering of the pH.

The following explanation is offered on the theory of this invention, it being understood however that the invention is not limited to this or any other specific theory of operation. It is thought that the higher solution viscosity normally obtained with wood pulp cellulose acetate as compared with cotton linters cellulose acetate is due to the existence of a greater number of carboxyl groups to which calcareous matter has attached itself. (Recently cellulose chemists have held that celluloses contain small amounts of carboxyl groups due to oxidation although the presence of carboxyl groups need not affect the molecular chain length.) While the invention is applicable to the washing of freshly formed organic esters of cellulose produced from cotton linters, the improvement or decrease in solution viscosity obtainable in this case is not nearly so marked because there are fewer carboxyl groups present in cotton linters cellulose and in the cellulose acetate produced therefrom for the calcareous matter to attach itself. When the Washing of the cellulose acetate is carried out in accordance with this invention, it is believed that the hydrogen ion concentration is such that the magnesium and calcium ions present in the water will selectivel displace the hydrogen of the sulfuric acid half-ester groups without replacing the hydrogen of the carboxyl groups. However, if the pH of the wash water is somewhat above 4.5, the hydrogen ion concentration is insufficient to prevent substantially complete displacement of hydrogen of the carboxyl groups by magnesium or calcium in the water and, consequently, the ash alkalinity of the cellulose acetate is increased, as is the viscosity. On the other hand. the sulfuric acid half-ester groups are not affected and the stability of the cellulose ester remains good. When the pH of the wash water drops below 4.5. for instance to 4.2. the hydrogen ion concentration is sufliciently great to prevent not only a neutralization of the carboxyl groups. but also prevents displacement of some of the hydrogens on the sulfuric acid half-ester groups by alkaline earth metal ions. The loss of alkaline earth metal atoms or prevention of alkaline earth metal atoms from attaching themselves to the sulfate half-ester groups is readily apparent by the fact that the stabilit of the cellulose e t r drops rapidly and the ash alkalinity decreases much more rapidly than before.

Although, as a matter of convenience, this invention has been described with particular reference to the reduction in viscosity of acetonesoluble cellulose acetate, it is not so limited but is applicable also to working viscosity reduction of other cellulose esters, such as cellulose propionate, cellulose butyrate, cellulose acetopropionate, cellulose acetobutyrate, and the like.

This invention enables effective reduction in viscosity of cellulose ester solution so that the solution can be readily spun in a continuous manner without difficulty to produce filaments, yarns and the like with desirable characteristics. By means of this invention, cellulose organic acid esters of increased average molecular chain length can be produced and spun to produce yarns of improved properties without difiiculty. Also, cellulose organic acid esters of substantially the same average molecular chain length as have long been used may be produced and dissolved to form solutions of higher concentration than are normally used without encountering excessive increase in solution viscosit and spun or cast to produce filaments, yarns, film and the like. This invention not only enables the working viscosity of cellulose organic acid esters to be greatly reduced, but the cellulose organic acid esters so prepared have inherently better properties as is evidenced by the lower ash alkalinity and improved stability.

As many widely different embodiments of this invention maybe made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.

I claim:

1. The process of reducing the solution viscosity of organic acid esters of cellulose which comprises washing a freshly precipitated cellulose organic acid ester containing combined mineral acid with water having a hardness of at least 20 parts per million calculated as calcium carbonate and containing sufficient of the same mineral acid as that combined with the cellulose organic acid ester to substantially lower the pH of said water to a value not less than about 4.5.

2. The process of reducing the solution viscosity of organic acid esters of cellulose which comprises washing a freshly precipitated cellulose organic acid ester containing combined mineral acid with water having a hardness of from about 60 to about parts per million calculated as calcium carbonate and containing suflicient of the same mineral acid as that combined with the cellulose organic acid ester to substantially lower the pH of said water to a value not less than about 4.5.

3. The process of reducing the solution viscosity of organic acid esters of cellulose which comprises washing a freshly precipitated cellulose organic acid ester containing combined mineral acid with water having a hardness of from about 60 to about 90 parts per million calculated as calcium carbonate and containing suflicient ofthe same mineral acid as that combined with the cellulose organic acid ester to substantially lower the pH of said water to a value of between about 4.5 and 5.5.

4. The process of reducing the solution viscosity of cellulose acetate which comprises washing a freshly precipitated cellulose acetate containing combined mineral acid with water having a hardness of at least 20 parts per million calculated as calcium carbonate and containing suflicient of the same mineral acid as that combined with the cellulose acetate to substantially lower the pH of said water to a value not less than about 4.5.

5. The process of reducing the olution viscosity of cellulose acetate which comprises washing a freshly precipitated cellulose acetate containing combined mineral acid with water having a hardness of from about 60 to about 90 parts per million calculated as calcium carbonate and containing sufiicient of the same mineral acid as that combined with the cellulose acetate to substantially lower the pH of said water to a. value of between about 4.5 and 5.5.

6. The process of reducing the solution viscosity of cellulose acetate which comprises washing a freshly precipitated cellulose acetate containing combined sulfuric acid with water having a hardness of at least 20 parts per million calculated as calcium carbonate and containing sufilcient sulfuric acid to substantially lower the pH of said water to a value of not less than about 4.5.

7. The process of reducing the solution viscosity 'of cellulose acetate which comprises washing a freshly precipitated cellulose acetate containing combined sulfuric acid with water having a hardnes of from about 60 to about 90 parts per million calculated as calcium carbonate and containing sufiicient' sulfuric acid to substantially lower the pH of said water to a value of between about 4.5 and 5.5.

8. The process of reducing the solution viscosity of organic acid esters of cellulose which comprises washing a freshly precipitated cellulose organic acid ester produced by the acylation of wood pulp and containing combined mineral acid, with water having a hardness of at least 20 parts per million calculated as calcium carbonate and containing suflicient of the same mineral acid as that combined with the cellulose organic acid ester to substantially lower the pH of said water to a value not less than about 4.5.

9. The proces of reducing the solution viscosity of organic acid esters of cellulose which comprises washing a freshly precipitated cellulose organic acid ester produced by the acylation of wood pulp and containing combined mineral acid, with water having a hardness of from about 60 to about 90 parts per million calculated as calcium car- I bonate and containing sufiicient of the same mineral acid as that combined with the cellulose organic acid ester to substantially lower the pH of said water to a value "of between about4.5 and 5.5.

10. The process of reducing the solution viscosity of cellulose acetate which comprises washing a freshly precipitated cellulose acetate produced by the acylation of wood pulp cellulose and containing combined mineral acid with water having a hardness of from about 60 to about 90 parts per million calculated as calcium carbonate and containing sufllcient of the same mineral acid as that combined with the cellulose acetate to substantially lower the pH of said water to :a value of between about 4.5 and 5.5.

11. The process of reducing the solution viscosity of cellulose acetate which comprises: washing a freshly precipitated cellulose acetate produced by the acylation of wood pulp cellulose and containing combined sulfuric acid with water having a hardness of from about 60 to about 90 parts per million calculated as calcium carbonate and containing sufficient sulfuric acid as to substantially lower the pH of said water to a value of between about 4.5 and 5.5.

RICHARD wnsoN NEBEL. 

